Implantable polymeric materials capable of being degraded and absorbed by the body have been in use for many decades. These biodegradable materials are often used as structural supports, such as scaffolds for guiding tissue regeneration, as sutures, staples, and meshes, as protective barriers during wound healing, or as a means for delivering therapeutical substances to a recipient in a controlled fashion.
The degradation of these polymers in vivo may occur through a variety of mechanisms. For example, the covalent linkages in a biodegradable polymer may be labile to enzymatic cleavage or non-enzymatic hydrolysis. The product of the hydrolysis may or may not be soluble in water. Water-soluble products are often excreted directly from the body or are removed from the body after passing through a particular metabolic pathway. For water-insoluble products, the phagocytotic action of cells, such as macrophages and/or foreign body giant cells, may play a major part in degrading and eliminating the products. Such phagocytotic biodegradation usually involves the invocation of an inflammatory response to the biodegradable material by the recipient.
When biodegradable materials are implanted in a recipient and used as structural supports, there is often an ingrowth of cells from the recipient into the space occupied by the material as the material degrades and is removed from the body of the recipient. In some circumstances, fairly complex, three-dimensional, tissue structures can be grown this way. One example is the regeneration of cartilage, which is accomplished by pre-seeding a degradable fiber mesh with chondrocytes, and after several weeks of culturing the cells in vitro, implanting the constructs in a recipient (see U.S. Pat. No. 5,041,138, issued to Vacanti, et. al., for example). Another example is the regeneration of dermis using fibroblasts seeded on a degradable polymer scaffold (see U.S. Pat. No. 5,443,950, issued to Naughton, et. al., for example).
To effect controlled release of a therapeutical agent from a biodegradable polymer, the therapeutical agent is usually admixed with the particular biodegradable polymer during manufacture of the controlled release material. Following implantation of the material in a recipient, the therapeutical agent is released from the biodegradable polymer as the polymer is degraded by the body of the recipient (see U.S. Pat. No. 4,389,330, issued to Tice et al., for example). The rate of degradation of a biodegradable polymer is dependent on the chemical composition of the polymer, the crystallinity of the sample, the porosity, and the wettability. For example, hydrophobic biodegradable polymers usually degrade at a slower rate than hydrophilic biodegradable polymers.
Since most biodegradable materials, such as poly(alpha-hydroxy esters), are relatively hydrophobic, it may be desirable to modify the materials to render the surfaces thereof more hydrophilic. By making the surfaces more hydrophilic, the degradation rate of the polymer may be increased, cell attachment may be enhanced, or protein deposition patterns may be altered in such a manner as to improve the biocompatibility or cell response to the polymer.
Hydrophobic surfaces are low energy surfaces that are readily wetted by low surface tension fluids, such as low molecular weight hydrocarbons or alcohols, and most low molecular weight organic solvents, such as benzene, acetone, toluene, and dioxane, etc. Hydrophilic surfaces, on the other hand, are high energy surfaces that are readily wetted by high surface tension fluids. Examples of high surface tension fluids include, but are not limited to, liquid water, aqueous salt and protein solutions, dimethyl formamide, dimethyl sulfoxide, glycerol, hexamethyl phosphorictriamide, formamide, and ethylene glycol, for example.
Table 1 lists examples of polymeric materials in order of increasing surface tension, with representative values of the surface tension (dyn/cm) for each material measured at 20.degree. C. (Polymer Handbook, 3rd Edition, J. Brandrup, E. H. Immergut, Eds., John Wiley & Sons, Inc., pp. VI411-VI426, 1989). In general, the surface tension of polymeric materials ranges from about 10 to 70 dyn/cm. Many polymers have intermediate surface energies and the wetting behavior of high surface tension fluids on these polymers is dependent on factors such as functional groups, surface roughness, contamination, and surface mobility in addition to the surface tension of the polymer surface.
TABLE 1 ______________________________________ Surface Tension Polymer (dyn/cm) ______________________________________ poly(hexafluoropropylene) 17 poly(dimethyl siloxane) 20 poly(tetrafluoroethylene) 24 poly(trifluoroethylene) 27 poly(vinylidine fluoride) 33 poly(vinyl alcohol) 37 poly(styrene) 40 poly(methyl methacrylate) 41 poly(vinyl chloride) 42 poly(ethylene terephthalate) poly(hydroxyethyl methacrylate) (40% water) 69 ______________________________________ Source: Polymer Handbook, 3rd Edition, J. Brandrup, E.H. Immergut, Eds., John Wiley & Sons, Inc., pp. VI411-V426, 1989. Values were determined at 20.degree. C.
One method to compare the hydrophobicity of a non-porous, solid surface of one material with the non-porous, solid surface of another material is to orient the material horizontally and apply a droplet of distilled water to the surface of the material. The angle which the edge of the water droplet makes with the surface is the advancing contact angle or simply the "contact angle." For most hydrophobic materials, the contact angle will be above 90.degree.. For example, the contact angle of water on poly(tetrafluoroethylene) is approximately 120.degree.. For most hydrophilic materials, the contact angle will be below about 30.degree.. For example, the contact angle of water on poly(hydroxyethyl methacrylate) is approximately 15.degree.. For the purposes of this invention, solid materials which have been modified with one or more layers of hydrophilic polymers will be considered having been rendered hydrophilic if the contact angle decreases by 10.degree. or more. A preferred result would be a resulting contact angle less than 30.degree..
For porous materials, a simple test to compare the wettability of one material with another is to position the material horizontally and apply a droplet of distilled water onto the surface of the material. For most hydrophobic, porous materials, the water droplet will remain on the surface. For most hydrophilic, porous materials, the water droplet will immediately penetrate into the pores of the sample. The fibers or polymer strands which form the sides of the pores act as hydrophilic surfaces which the water spreads on. The pores attract the water droplet by capillary action. For the purposes of this invention, porous materials which wet within 1 second after exposure to a droplet of water are considered hydrophilic. Porous materials which do not spontaneously wet, which require more than 1 second to wet, or which require mechanical agitation to thoroughly wet, are considered hydrophobic.
It is known to treat non-biodegradable materials, such as polytetrafluoroethylene, with surfactants or other hydrophilic polymers to render the surfaces of these materials more hydrophilic and often wettable with liquid water. Such a surfactant treatment is often unstable, however, with the surfactant easily leaching from the hydrophobic material when in use. A more stable surfactant coating can be made on hydrophobic materials by cross-linking the components of the surfactant together on the material (see U.S. Pat. No. 4,113,912, issued to Okita, for example).
A stable coating of a hydrophilic material on a hydrophobic biodegradable material would be undesirable, however, because the cross-linked hydrophilic material would most likely remain intact in a recipient after the biodegradable material to which it was initially applied had degraded and been removed from the body of the recipient. For example, a method to render biodegradable polymers more wettable has been described by Mooney, et. al. (Mooney, D. J., Park, S., Kaufmann, P. M., Sano, K., McNamara, K., Vacanti, J. P., Langer, R., "Biodegradable sponges for hepatocyte transplantation", J. Biomed. Mat. Res., 29:959-965, (1995)). Porous sponges fabricated from poly(L-lactic acid) (PLA) were rendered more hydrophilic by adding the surfactant poly(vinyl alcohol) (PVA) to the interior surfaces of the porous PLA sponges. The addition of PVA increased the wettability of the polymer sponge and resulted in a more thorough infiltration and a more extensive seeding of the sponge with hepatocytes than occurred with untreated sponges. Mooney, et. al. also described the use of commercially available PVA sponges as cell transplantation devices, but this approach was discounted by Mooney, et. al. as unsuitable due to the non-degradable nature of covalently cross-linked PVA. Methods to reversibly cross-link or otherwise transiently stabilize the PVA coating on the PLA sponge were not described.
The hydrophobicity of biodegradable polymers also presents a problem when it is desired to immobilize a bioactive species onto the surface of a device made from a biodegradable material, rather than incorporate the bioactive species into the biodegradable material. In the simplest method, for example, a bioactive species is immobilized onto the surface of a biodegradable polymer via simple physicochemical adsorption (physisorption). However, physisorption of bioactive species is often kinetically and thermodynamically unstable, highly reversible, and competitively displaced by solution phase reactants, products, or nutrients. Thus, physisorption of bioactive species to biodegradable materials is not usually a suitable immobilization technique.
The term "immobilize," and its derivatives, as used herein refers to the attachment of a bioactive species directly to a biodegradable support member or to a biodegradable support member through at least one intermediate component. As used herein, the term "attach" and its derivatives refer to adsorption, such as, physisorption, or chemisorption, ligand/receptor interaction, covalent bonding, hydrogen bonding, or ionic bonding of a polymeric substance or a bioactive species to a biodegradable support member.
"Bioactive species" include enzymes, organic catalysts, ribozymes, organometallics, proteins, glycoproteins, peptides, polyamino acids, antibodies, nucleic acids, steroidal molecules, antibiotics, antimycotics, cytokines, carbohydrates, oleophobics, lipids, extracellular matrix and/or its individual components, pharmaceuticals, and therapeutics, for example. Cells, such as, mammalian cells, reptilian cells, amphibian cells, avian cells, insect cells, planktonic cells, cells from non-mammalian marine vertebrates and invertebrates, plant cells, microbial cells, protists, genetically engineered cells, and organelles, such as mitochondria, are also bioactive species. In addition, non-cellular biological entities, such as viruses, virenos, and prions are considered bioactive species.
Bioactive species could be attached to a hydrophobic biodegradable polymer through chemically functional groups on the components of the polymer. However, many biodegradable polymers lack free chemically functional groups altogether or have such reduced numbers that significant quantities of bioactive species cannot be immobilized thereto. For example, the biodegradable polymers poly(lactic acid) and poly(glycolic acid) do not contain any chemically functional groups along the hydrocarbon backbone of the materials to which a bioactive species can be covalently coupled. One strategy that has been proposed for introducing functional groups into poly(lactic acid) is the copolymerization of lactide with a cyclic monomer of lactic acid and the amino acid lysine to create poly(lactic acid-co-lysine) (see U.S. Pat. No. 5,399,665, issued to Barrera, et. al.). This copolymer provides side chains that terminate in amino (NH.sub.2) groups. These amino groups can be used as attachment sites for the immobilization of bioactive species. Since this method chemically alters the polymer, many of the properties of the polymer are subject to change. For example, the degradation rate and the tensile strength may be effected by the alteration to the polymer. The processing of the polymer may also be effected by altering the hydrocarbon backbone of the polymer.
Prisell et al., in U.S. Pat. No. 5,740,829, have attempted to immobilize proteins onto the surface of a biodegradable material by first physisorbing the protein onto the biodegradable material, followed by cross-linking the proteins together on the surface. In the method, proteins, such as bone morphogenetic protein (BMP) or insulin-like growth factor-1-receptor (IGF-1 receptor), were adsorbed onto biodegradable polymers, such as poly(glycolic acid) (PGA) or poly(lactic acid) (PLA), and cross-linked in place using imidocarbonates, carbonates, oxiranes, aziridine, activated double bonds, or halogens. This method often results in a very inefficient immobilization, however. In addition, the cross-linked proteins often have a marked decrease in bioactivity. Accordingly, this approach is usually unsuitable for immobilization of bioactive species to a biodegradable support member.
Stable immobilization of bioactive species onto a hydrophobic support member, such as porous polytetrafluoroethylene, is taught by Drumheller in U.S. patent application Ser. No. 08/660,698, filed Jun. 3, 1996. In this method, hydrophobic surfaces, inter alia, are rendered hydrophilic and wettable with liquid water by attaching a surfactant material to the hydrophobic surface and cross-linking the surfactant together, forming a first layer thereon. Additional layers of hydrophilic polymers are attached to the first layer to amplify the number of chemically functional groups available for the subsequent immobilization of bioactive species thereto. It would be undesirable to stably immobilize a bioactive species on an implantable biodegradable material according to the method of Drumheller because the bioactive species and its immobilization scaffold will most often remain intact in a recipient after the biodegradable support it has been applied to has degraded and been removed from the body of the recipient.
A normally hydrophobic biodegradable material having surfaces that are rendered more hydrophilic with a hydrophilic material that is initially stable on the surface of the biodegradable material, but is itself subject to degradation upon implantation in a recipient would be useful. Such a material with bioactive species immobilized thereto would also be useful. Ideally, constructs used to render hydrophobic biodegradable materials hydrophilic and amenable to immobilization of bioactive species thereto would be transitory in nature and not remain intact in the body of a recipient substantially longer than the biodegradable material. There is a need, therefore, for a biodegradable material having hydrophobic surfaces that are rendered more hydrophilic with hydrophilic polymeric materials that are biodegradable and to which bioactive species can be readily immobilized.